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The theoretical treatment of complex oxide structures requires a combination of efficient methods to calculate structural, electronic, and magnetic properties, due to special challenges such as strong correlations and disorder. In terms of a multicode approach, this study combines various complementary first‐principles methods based on density functional theory to exploit their specific strengths. Pseudopotential methods, known for giving reliable forces and total energies, are used for structural optimization. The optimized structure serves as input for the Green's function and linear muffin‐tin orbital methods. Those methods are powerful for the calculation of magnetic ground states and spectroscopic properties. Within the multicode approach, disorder is investigated by means of the coherent potential approximation within a Green's function method or by construction of special quasirandom structures in the framework of the pseudopotential methods. Magnetic ground states and phase transitions are studied using an effective Heisenberg model treated in terms of a Monte Carlo method, where the magnetic exchange parameters are calculated from first‐principles. The performance of the multicode approach is demonstrated with different examples, including defect formation, strained films, and surface properties.

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We present the first principles investigation of the electronic structure and physical properties of doped lithium nitridometalates Li2(Li1−xMx)N (LiMN) with M = Cr, Mn, Fe, Co, and Ni. The diverse properties include the equilibrium magnetic moments, magneto-crystalline anisotropy, magneto-optical Kerr spectra and x-ray magnetic circular dichroism. We explain the huge magnetic anisotropy in LiFeN by its unique electronic structure which ultimately leads to a series of unusual physical properties. The most unique property is a complete suppression of relativistic effects and freezing of orbital moments for in-plane orientation of the magnetization. This leads to a huge spatial anisotropy of many magnetic properties including energy, Kerr and dichroism effects. LiFeN is identified as an ultimate single-ion anisotropy system where a nearly insulating state can be produced by a spin orbital coupling alone. A very non-trivial strongly fluctuating and sign changing character of the magnetic anisotropy with electronic doping is predicted theoretically. A very anisotropic and large Kerr effect due to the interband transitions between atomic like Fe 3d bands is found for LiFeN. A very strong anisotropy of the X-ray magnetic circular dichroism for the Fe K spectrum and a very weak one for the Fe L2,3 spectra in LiFeN is also predicted.

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First-principles band-structure calculations of the magneto-optical Kerr spectra of MnBi and related compounds are reported. We find that band-structure theory, based on density-functional theory in the local spin-density approximation, explains the measured Kerr effect of MnBi very well. A giant Kerr rotation of about −1.75° at 1.8 eV photon energy is given by our ab initio calculations, in accordance with recent experiments. A second peak at 3.4 eV in the Kerr rotation spectrum, however, comes out smaller in our calculations than what was recently measured. It is discussed that this can be due to the Mn–Bi stoichiometry. The microscopic origin of the giant Kerr effect in MnBi is analyzed in detail. We find that the huge Kerr effect in MnBi is caused by the combination of a sizeable magnetic moment of 3.7 μB on manganese, the large spin-orbit coupling of bismuth, and a strong hybridization between the manganese d bands and the bismuth p states. The magneto-optically active states are mainly the p states of Bi. We pay further attention to the experimentally observed unusual temperature dependence of the MnBi Kerr spectra. We show that the observed temperature dependence can be explained by the reduction of the magnetic moment and the average lifetime with increasing temperature. The ab initio calculated Kerr effect in MnBi is furthermore compared to that calculated for the isoelectronic compounds MnAs and MnSb, and that of CrBi, CrTe, and Mn2Bi.

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